目录

This tutorial teaches you how to use collections that implement the Iterable class—for example List and Set. Iterables are basic building blocks for all sorts of Dart applications, and you're probably already using them, even without noticing. This tutorial helps you make the most out of them.

Using the embedded DartPad editors, you can test your knowledge by running example code and completing exercises.

To get the most out of this tutorial, you should have basic knowledge of Dart syntax.

This tutorial covers the following material:

  • How to read elements of an Iterable.
  • How to check if the elements of an Iterable satisfy a condition.
  • How to filter the contents of an Iterable.
  • How to map the contents of an Iterable to a different value.

Estimated time to complete this tutorial: 60 minutes.

The exercises in this tutorial have partially completed code snippets. You can use DartPad to test your knowledge by completing the code and clicking the Run button. Don't edit the test code in the main function or below.

If you need help, expand the Hint or Solution dropdown after each exercise.

What are collections?

#

A collection is an object that represents a group of objects, which are called elements. Iterables are a kind of collection.

A collection can be empty, or it can contain many elements. Depending on the purpose, collections can have different structures and implementations. These are some of the most common collection types:

  • List: Used to read elements by their indexes.
  • Set: Used to contain elements that can occur only once.
  • Map: Used to read elements using a key.

What is an Iterable?

#

An Iterable is a collection of elements that can be accessed sequentially.

In Dart, an Iterable is an abstract class, meaning that you can't instantiate it directly. However, you can create a new Iterable by creating a new List or Set.

Both List and Set are Iterable, so they have the same methods and properties as the Iterable class.

A Map uses a different data structure internally, depending on its implementation. For example, HashMap uses a hash table in which the elements (also called values) are obtained using a key. Elements of a Map can also be read as Iterable objects by using the map's entries or values property.

This example shows a List of int, which is also an Iterable of int:

dart
Iterable<int> iterable = [1, 2, 3];

The difference with a List is that with the Iterable, you can't guarantee that reading elements by index will be efficient. Iterable, as opposed to List, doesn't have the [] operator.

For example, consider the following code, which is invalid:

baddart
Iterable<int> iterable = [1, 2, 3];
int value = iterable[1];

If you read elements with [], the compiler tells you that the operator '[]' isn't defined for the class Iterable, which means that you can't use [index] in this case.

You can instead read elements with elementAt(), which steps through the elements of the iterable until it reaches that position.

dart
Iterable<int> iterable = [1, 2, 3];
int value = iterable.elementAt(1);

Continue to the next section to learn more about how to access elements of an Iterable.

Reading elements

#

You can read the elements of an iterable sequentially, using a for-in loop.

Example: Using a for-in loop

#

The following example shows you how to read elements using a for-in loop.

void main() {
  const iterable = ['Salad', 'Popcorn', 'Toast'];
  for (final element in iterable) {
    print(element);
  }
}

Example: Using first and last

#

In some cases, you want to access only the first or the last element of an Iterable.

With the Iterable class, you can't access the elements directly, so you can't call iterable[0] to access the first element. Instead, you can use first, which gets the first element.

Also, with the Iterable class, you can't use the operator [] to access the last element, but you can use the last property.

void main() {
  Iterable<String> iterable = const ['Salad', 'Popcorn', 'Toast'];
  print('The first element is ${iterable.first}');
  print('The last element is ${iterable.last}');
}

In this example you saw how to use first and last to get the first and last elements of an Iterable. It's also possible to find the first element that satisfies a condition. The next section shows how to do that using a method called firstWhere().

Example: Using firstWhere()

#

You already saw that you can access the elements of an Iterable sequentially, and you can easily get the first or last element.

Now, you learn how to use firstWhere() to find the first element that satisfies certain conditions. This method requires you to pass a predicate, which is a function that returns true if the input satisfies a certain condition.

dart
String element = iterable.firstWhere((element) => element.length > 5);

For example, if you want to find the first String that has more than 5 characters, you must pass a predicate that returns true when the element size is greater than 5.

Run the following example to see how firstWhere() works. Do you think all the functions will give the same result?

bool predicate(String item) {
  return item.length > 5;
}

void main() {
  const items = ['Salad', 'Popcorn', 'Toast', 'Lasagne'];

  // You can find with a simple expression:
  var foundItem1 = items.firstWhere((item) => item.length > 5);
  print(foundItem1);

  // Or try using a function block:
  var foundItem2 = items.firstWhere((item) {
    return item.length > 5;
  });
  print(foundItem2);

  // Or even pass in a function reference:
  var foundItem3 = items.firstWhere(predicate);
  print(foundItem3);

  // You can also use an `orElse` function in case no value is found!
  var foundItem4 = items.firstWhere(
    (item) => item.length > 10,
    orElse: () => 'None!',
  );
  print(foundItem4);
}

In this example, you can see three different ways to write a predicate:

  • As an expression: The test code has one line that uses arrow syntax (=>).
  • As a block: The test code has multiple lines between brackets and a return statement.
  • As a function: The test code is in an external function that's passed to the firstWhere() method as a parameter.

There is no right or wrong way. Use the way that works best for you, and that makes your code easier to read and understand.

The final example calls firstWhere() with the optional named parameter orElse, which provides an alternative when an element isn't found. In this case, the text 'None!' is returned because no element satisfies the provided condition.

Exercise: Practice writing a test predicate

#

The following exercise is a failing unit test that contains a partially complete code snippet. Your task is to complete the exercise by writing code to make the tests pass. You don't need to implement main().

This exercise introduces singleWhere() This method works similarly to firstWhere(), but in this case it expects only one element of the Iterable to satisfy the predicate. If more than one or no element in the Iterable satisfies the predicate condition, then the method throws a StateError exception.

Your goal is to implement the predicate for singleWhere() that satisfies the following conditions:

  • The element contains the character 'a'.
  • The element starts with the character 'M'.

All the elements in the test data are strings; you can check the class documentation for help.

// Implement the predicate of singleWhere
// with the following conditions
// * The element contains the character `'a'`
// * The element starts with the character `'M'`
String singleWhere(Iterable<String> items) {
  return items.singleWhere(TODO('Implement the outlined predicate.'));
}

// The following code is used to provide feedback on your solution.
// There is no need to read or modify it.
void main() {
  const items = [
    'Salad',
    'Popcorn',
    'Milk',
    'Toast',
    'Sugar',
    'Mozzarella',
    'Tomato',
    'Egg',
    'Water',
  ];

  try {
    final str = singleWhere(items);
    if (str == 'Mozzarella') {
      print('Success. All tests passed!');
    } else {
      print(
        'Tried calling singleWhere, but received $str instead of '
        'the expected value \'Mozzarella\'',
      );
    }
  } on StateError catch (stateError) {
    print(
      'Tried calling singleWhere, but received a StateError: ${stateError.message}. '
      'singleWhere will fail if 0 or many elements match the predicate.',
    );
  } on UnimplementedError {
    print(
      'Tried running `singleWhere`, but received an error. '
      'Did you implement the function?',
    );
  } catch (e) {
    print('Tried calling singleWhere, but received an exception: $e');
  }
}
Hint

Your solution might make use of the contains and startsWith methods from the String class.

Solution
dart
String singleWhere(Iterable<String> items) {
  return items.singleWhere(
          (element) => element.startsWith('M') && element.contains('a'));
}

Checking conditions

#

When working with Iterable, sometimes you need to verify that all the elements of a collection satisfy some condition.

You might be tempted to write a solution using a for-in loop like this one:

baddart
for (final item in items) {
  if (item.length < 5) {
    return false;
  }
}
return true;

However, you can accomplish the same using the every() method:

dart
return items.every((item) => item.length >= 5);

Using the every() method results in code that is more readable, compact, and less error-prone.

Example: Using any() and every()

#

The Iterable class provides two methods that you can use to verify conditions:

  • any(): Returns true if at least one element satisfies the condition.
  • every(): Returns true if all elements satisfy the condition.

Run this exercise to see them in action.

void main() {
  const items = ['Salad', 'Popcorn', 'Toast'];

  if (items.any((item) => item.contains('a'))) {
    print('At least one item contains "a"');
  }

  if (items.every((item) => item.length >= 5)) {
    print('All items have length >= 5');
  }
}

In the example, any() verifies that at least one element contains the character a, and every() verifies that all elements have a length equal to or greater than 5.

After running the code, try changing the predicate of any() so it returns false:

dart
if (items.any((item) => item.contains('Z'))) {
  print('At least one item contains "Z"');
} else {
  print('No item contains "Z"');
}

You can also use any() to verify that no element of an Iterable satisfies a certain condition.

Exercise: Verify that an Iterable satisfies a condition

#

The following exercise provides practice using the any() and every() methods, described in the previous example. In this case, you work with a group of users, represented by User objects that have the member field age.

Use any() and every() to implement two functions:

  • Part 1: Implement anyUserUnder18().
    • Return true if at least one user is 17 or younger.
  • Part 2: Implement everyUserOver13().
    • Return true if all users are 14 or older.
bool anyUserUnder18(Iterable<User> users) {
  // TODO: Implement the anyUserUnder18 function.
}

bool everyUserOver13(Iterable<User> users) {
  // TODO: Implement the everyUserOver13 function.
}

class User {
  final String name;
  final int age;

  User(
    this.name,
    this.age,
  );
}

// The following code is used to provide feedback on your solution.
// There is no need to read or modify it.
void main() {
  final users = [
    User('Alice', 21),
    User('Bob', 17),
    User('Claire', 52),
    User('David', 14),
  ];

  try {
    final out = anyUserUnder18(users);
    if (!out) {
      print('Looks like `anyUserUnder18` is wrong. Keep trying!');
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `anyUserUnder18`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print('Tried running `anyUserUnder18`, but received an exception: $e');
    return;
  }

  try {
    // with only one user older than 18, should be false
    final out = anyUserUnder18([User('Alice', 21)]);
    if (out) {
      print(
          'Looks like `anyUserUnder18` is wrong. What if all users are over 18?');
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `anyUserUnder18`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
      'Tried running `anyUserUnder18([User("Alice", 21)])`, '
      'but received an exception: $e',
    );
    return;
  }

  try {
    final out = everyUserOver13(users);
    if (!out) {
      print(
        'Looks like `everyUserOver13` is wrong. '
        'There are no users under 13!',
      );
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `everyUserOver13`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
      'Tried running `everyUserOver13`, '
      'but received an exception: $e',
    );
    return;
  }

  try {
    final out = everyUserOver13([User('Dan', 12)]);
    if (out) {
      print(
        'Looks like `everyUserOver13` is wrong. '
        'There is at least one user under 13!',
      );
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `everyUserOver13`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
      'Tried running `everyUserOver13([User(\'Dan\', 12)])`, '
      'but received an exception: $e',
    );
    return;
  }

  print('Success. All tests passed!');
}
Hint

Remember to use the any and every methods from the Iterable class. For help and examples using these methods, refer to the earlier discussion of them.

Solution
dart
bool anyUserUnder18(Iterable<User> users) {
  return users.any((user) => user.age < 18);
}

bool everyUserOver13(Iterable<User> users) {
  return users.every((user) => user.age > 13);
}

Filtering

#

The previous sections cover methods like firstWhere() or singleWhere() that can help you find an element that satisfies a certain predicate.

But what if you want to find all the elements that satisfy a certain condition? You can accomplish that using the where() method.

dart
var evenNumbers = numbers.where((number) => number.isEven);

In this example, numbers contains an Iterable with multiple int values, and where() finds all the numbers that are even.

The output of where() is another Iterable, and you can use it as such to iterate over it or apply other Iterable methods. In the next example, the output of where() is used directly inside the for-in loop.

dart
var evenNumbers = numbers.where((number) => number.isEven);
for (final number in evenNumbers) {
  print('$number is even');
}

Example: Using where()

#

Run this example to see how where() can be used together with other methods like any().

void main() {
  var evenNumbers = const [1, -2, 3, 42].where((number) => number.isEven);

  for (final number in evenNumbers) {
    print('$number is even.');
  }

  if (evenNumbers.any((number) => number.isNegative)) {
    print('evenNumbers contains negative numbers.');
  }

  // If no element satisfies the predicate, the output is empty.
  var largeNumbers = evenNumbers.where((number) => number > 1000);
  if (largeNumbers.isEmpty) {
    print('largeNumbers is empty!');
  }
}

In this example, where() is used to find all numbers that are even, then any() is used to check if the results contain a negative number.

Later in the example, where() is used again to find all numbers larger than 1000. Because there are none, the result is an empty Iterable.

Example: Using takeWhile

#

The methods takeWhile() and skipWhile() can also help you filter elements from an Iterable.

Run this example to see how takeWhile() and skipWhile() can split an Iterable containing numbers.

void main() {
  const numbers = [1, 3, -2, 0, 4, 5];

  var numbersUntilZero = numbers.takeWhile((number) => number != 0);
  print('Numbers until 0: $numbersUntilZero');

  var numbersStartingAtZero = numbers.skipWhile((number) => number != 0);
  print('Numbers starting at 0: $numbersStartingAtZero');
}

In this example, takeWhile() returns an Iterable that contains all the elements before the one that satisfies the predicate. On the other hand, skipWhile() returns an Iterable that contains all elements after and including the first one that doesn't satisfy the predicate.

After running the example, change takeWhile() to take elements until it reaches the first negative number.

dart
var numbersUntilNegative =
    numbers.takeWhile((number) => !number.isNegative);

Notice that the condition number.isNegative is negated with !.

Exercise: Filtering elements from a list

#

The following exercise provides practice using the where() method with the class User from the previous exercise.

Use where() to implement two functions:

  • Part 1: Implement filterOutUnder21().
    • Return an Iterable containing all users of age 21 or more.
  • Part 2: Implement findShortNamed().
    • Return an Iterable containing all users with names of length 3 or less.
Iterable<User> filterOutUnder21(Iterable<User> users) {
  // TODO: Implement the filterOutUnder21 function.
}

Iterable<User> findShortNamed(Iterable<User> users) {
  // TODO: Implement the findShortNamed function.
}

class User {
  final String name;
  final int age;

  User(
    this.name,
    this.age,
  );
}

// The following code is used to provide feedback on your solution.
// There is no need to read or modify it.
void main() {
  final users = [
    User('Alice', 21),
    User('Bob', 17),
    User('Claire', 52),
    User('Dan', 12),
  ];

  try {
    final out = filterOutUnder21(users);
    if (out.any((user) => user.age < 21) || out.length != 2) {
      print(
        'Looks like `filterOutUnder21` is wrong, there are '
        'exactly two users with age under 21. Keep trying!',
      );
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `filterOutUnder21`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
      'Tried running `filterOutUnder21`, '
      'but received an exception: ${e.runtimeType}',
    );
    return;
  }

  try {
    final out = findShortNamed(users);
    if (out.any((user) => user.name.length > 3) || out.length != 2) {
      print(
        'Looks like `findShortNamed` is wrong, there are '
        'exactly two users with a three letter name. Keep trying!',
      );
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `findShortNamed`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
      'Tried running `findShortNamed`, '
      'but received an exception: ${e.runtimeType}',
    );
    return;
  }

  print('Success. All tests passed!');
}
Hint

Remember to take advantage of the where method from the Iterable class. For help and examples using where, refer to the earlier discussion of it.

Solution
dart
Iterable<User> filterOutUnder21(Iterable<User> users) {
  return users.where((user) => user.age >= 21);
}

Iterable<User> findShortNamed(Iterable<User> users) {
  return users.where((user) => user.name.length <= 3);
}

Mapping

#

Mapping Iterables with the method map() enables you to apply a function over each of the elements, replacing each element with a new one.

dart
Iterable<int> output = numbers.map((number) => number * 10);

In this example, each element of the Iterable numbers is multiplied by 10.

You can also use map() to transform an element into a different object—for example, to convert all int to String, as you can see in the following example:

dart
Iterable<String> output = numbers.map((number) => number.toString());

Example: Using map to change elements

#

Run this example to see how to use map() to multiply all the elements of an Iterable by 2. What do you think the output will be?

void main() {
  var numbersByTwo = const [1, -2, 3, 42].map((number) => number * 2);
  print('Numbers: $numbersByTwo');
}

Exercise: Mapping to a different type

#

In the previous example, you multiplied the elements of an Iterable by 2. Both the input and the output of that operation were an Iterable of int.

In this exercise, your code takes an Iterable of User, and you need to return an Iterable that contains strings containing each user's name and age.

Each string in the Iterable must follow this format: '{name} is {age}'—for example 'Alice is 21'.

Iterable<String> getNameAndAges(Iterable<User> users) {
  // TODO: Implement the getNameAndAges function.
}

class User {
  final String name;
  final int age;

  User(
    this.name,
    this.age,
  );
}

// The following code is used to provide feedback on your solution.
// There is no need to read or modify it.
void main() {
  final users = [
    User('Alice', 21),
    User('Bob', 17),
    User('Claire', 52),
  ];

  try {
    final out = getNameAndAges(users).toList();
    if (!_listEquals(out, ['Alice is 21', 'Bob is 17', 'Claire is 52'])) {
      print(
        'Looks like `getNameAndAges` is wrong. Keep trying! '
        'The output was: $out',
      );
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `getNameAndAges`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print('Tried running the function, but received an exception: $e');
    return;
  }

  print('Success. All tests passed!');
}

bool _listEquals<T>(List<T>? a, List<T>? b) {
  if (a == null) return b == null;
  if (b == null || a.length != b.length) return false;
  for (var index = 0; index < a.length; index += 1) {
    if (a[index] != b[index]) return false;
  }
  return true;
}
Hint

Remember to take advantage of the map method from the Iterable class. For help and examples using map, refer to the earlier discussion of it.

To concatenate multiple values into a single string, consider using string interpolation.

Solution
dart
Iterable<String> getNameAndAges(Iterable<User> users) {
  return users.map((user) => '${user.name} is ${user.age}');
}

Exercise: Putting it all together

#

It's time to practice what you learned, in one final exercise.

This exercise provides the class EmailAddress, which has a constructor that takes a string. Another provided function is isValidEmailAddress(), which tests whether an email address is valid.

Constructor/function Type signature Description
EmailAddress() EmailAddress(String address) Creates an EmailAddress for the specified address.
isValidEmailAddress() bool isValidEmailAddress(EmailAddress) Returns true if the provided EmailAddress is valid.

Write the following code:

Part 1: Implement parseEmailAddresses().

  • Write the function parseEmailAddresses(), which takes an Iterable<String> containing email addresses, and returns an Iterable<EmailAddress>.
  • Use the method map() to map from a String to EmailAddress.
  • Create the EmailAddress objects using the constructor EmailAddress(String).

Part 2: Implement anyInvalidEmailAddress().

  • Write the function anyInvalidEmailAddress(), which takes an Iterable<EmailAddress> and returns true if any EmailAddress in the Iterable isn't valid.
  • Use the method any() together with the provided function isValidEmailAddress().

Part 3: Implement validEmailAddresses().

  • Write the function validEmailAddresses(), which takes an Iterable<EmailAddress> and returns another Iterable<EmailAddress> containing only valid addresses.
  • Use the method where() to filter the Iterable<EmailAddress>.
  • Use the provided function isValidEmailAddress() to evaluate whether an EmailAddress is valid.
Iterable<EmailAddress> parseEmailAddresses(Iterable<String> strings) {
  // TODO: Implement the parseEmailAddresses function.
}

bool anyInvalidEmailAddress(Iterable<EmailAddress> emails) {
  // TODO: Implement the anyInvalidEmailAddress function.
}

Iterable<EmailAddress> validEmailAddresses(Iterable<EmailAddress> emails) {
  // TODO: Implement the validEmailAddresses function.
}

class EmailAddress {
  final String address;

  EmailAddress(this.address);

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      other is EmailAddress && address == other.address;

  @override
  int get hashCode => address.hashCode;

  @override
  String toString() => 'EmailAddress{address: $address}';
}

// The following code is used to provide feedback on your solution.
// There is no need to read or modify it.
void main() {
  const input = [
    '[email protected]',
    'bobgmail.com',
    '[email protected]',
  ];

  const correctInput = ['[email protected]', '[email protected]'];

  bool _listEquals<T>(List<T>? a, List<T>? b) {
    if (a == null) return b == null;
    if (b == null || a.length != b.length) return false;
    for (var index = 0; index < a.length; index += 1) {
      if (a[index] != b[index]) return false;
    }
    return true;
  }

  final Iterable<EmailAddress> emails;
  final Iterable<EmailAddress> correctEmails;
  try {
    emails = parseEmailAddresses(input);
    correctEmails = parseEmailAddresses(correctInput);
    if (emails.isEmpty) {
      print(
        'Tried running `parseEmailAddresses`, but received an empty list.',
      );
      return;
    }
    if (!_listEquals(emails.toList(), [
      EmailAddress('[email protected]'),
      EmailAddress('bobgmail.com'),
      EmailAddress('[email protected]'),
    ])) {
      print('Looks like `parseEmailAddresses` is wrong. Keep trying!');
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `parseEmailAddresses`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
      'Tried running `parseEmailAddresses`, '
      'but received an exception: $e',
    );
    return;
  }

  try {
    final out = anyInvalidEmailAddress(emails);
    if (!out) {
      print(
        'Looks like `anyInvalidEmailAddress` is wrong. Keep trying! '
        'The result should be false with at least one invalid address.',
      );
      return;
    }
    final falseOut = anyInvalidEmailAddress(correctEmails);
    if (falseOut) {
      print(
        'Looks like `anyInvalidEmailAddress` is wrong. Keep trying! '
        'The result should be false with all valid addresses.',
      );
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `anyInvalidEmailAddress`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
        'Tried running `anyInvalidEmailAddress`, but received an exception: $e');
    return;
  }

  try {
    final valid = validEmailAddresses(emails);
    if (emails.isEmpty) {
      print('Tried running `validEmailAddresses`, but received an empty list.');
      return;
    }
    if (!_listEquals(valid.toList(), [
      EmailAddress('[email protected]'),
      EmailAddress('[email protected]'),
    ])) {
      print('Looks like `validEmailAddresses` is wrong. Keep trying!');
      return;
    }
  } on UnimplementedError {
    print(
      'Tried running `validEmailAddresses`, but received an error. '
      'Did you implement the function?',
    );
    return;
  } catch (e) {
    print(
      'Tried running the `validEmailAddresses`, '
      'but received an exception: $e',
    );
    return;
  }

  print('Success. All tests passed!');
}

bool isValidEmailAddress(EmailAddress email) {
  return email.address.contains('@');
}
Solution
dart
Iterable<EmailAddress> parseEmailAddresses(Iterable<String> strings) {
  return strings.map((s) => EmailAddress(s));
}

bool anyInvalidEmailAddress(Iterable<EmailAddress> emails) {
  return emails.any((email) => !isValidEmailAddress(email));
}

Iterable<EmailAddress> validEmailAddresses(Iterable<EmailAddress> emails) {
  return emails.where((email) => isValidEmailAddress(email));
}

What's next

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